The present invention relates to a sheet detaching device for detaching a copysheet from the photoconductive drum of an electrophotographic copying machine.
It is well known from the prior art that in an electrophotographic copying machine a photoconductive element is wound or coated on the periphery of a drum which is rotated around its own axis for carrying the element in succession under the action of the following stations:
Charging station: wherein an electrostatic charge is uniformly deposited on the surface of the photoconductive element; PA1 Exposure station: wherein the charged surface of the photoconductive element is exposed to the light image of the original to be reproduced, whereby a latent image of the original is formed on the photoconductive surface; PA1 Developing station: wherein toner particles are attracted by the photoconductive surface so rendering the image visible; PA1 Transfer station: wherein a copysheet of plain paper is carried in contact with the developed image while an electrostatic charge of opposite polarity with respect to the charge of the photoconductive surface is applied to the back of the sheet, whereby the toner particles are attracted to the copysheet of plain paper in image configuration; PA1 Sheeting detaching station: wherein the copysheet carrying the toner particles is detached from the photoconductive surface against the force of attraction of the charges of opposite polarity located on the photoconductor and on the sheet respectively.
The sheet detaching devices usually comprise a corona charging device supplied with A.C. current, which renders ineffective the electrostatic attraction between the copy paper and the photoconductors by charging the copy paper with a continuously variable charge sign.
It is also known to add to the above A.C. corona charging device some mechanical devices to aid the detaching of the sheet.
Among mechanical devices of the above type, detaching fingers are known having a first inoperative position and a second working position into which they can be switched upon actuation, and in which they are inserted between the leading edge of the copy sheet and the photoconductive surface, during the rotation of the drum, so picking off the copy sheet from the drum.
It is also known to mount the detaching fingers within the drum in such a way that in their inoperative position they are within the drum, while, in their operative position, they come out from the periphery of the drum and insert themselves between the leading edge of the copy sheet and the photoconductive surface of the drum at the time that the leading edge of the sheet arrives in contact with the drum and thus before the transfer of the toner from photoconductor to the copy sheet.
This last type of device suffers from the disadvantage of covering with the fingers a portion of the photoconductive surface and thus a corresponding portion of toner image is not transferred to the copy-sheet.
A detaching device is also known in which the fingers are mounted adjacent the drum in such a way that they slide on the photoconductive surface. This type of device has the advantage of not requiring a special mechanism for its actuation in synchronism with the rotation of the drum, but it has also the disadvantage of causing the scraping of the photoconductive surface, expecially when the photoconductive element is a web around the drum.